Data communications networks often include elements that are connected by a wireless link. Typically, a number of static or fixed wireless nodes such as access points may be deployed within a local zone to which mobile devices may form wireless connections, the connections typically conforming to an industry standard such as an IEEE 802.11 standard, for example IEEE 802.11a. Preferably the wireless access points are arranged in such a way as to give useful coverage in the zone or area of coverage concerned and are connected to a data network by wired or wireless links.
There are many applications in which it is necessary to know the location of a mobile terminal, and various systems have been proposed and implemented for location estimation of a terminal on the basis of measurements of radio frequency propagation. The known systems can be categorized according to whether or not they require a survey of the radio frequency environment to be carried out.
Systems for location estimation that do not require a survey include, for example, estimation of location by relative time of arrival of radio signals sent to or from fixed nodes in the network such as access points or base stations. The radio signals may be on the uplink or the downlink, that is to say the times of arrival may be measured at a terminal for signals sent from a plurality of fixed nodes, or at a plurality of fixed nodes for a signal sent from the terminal. Alternatively, it is known to estimate location based on the signal strength received at fixed nodes from a terminal or at the terminal from a plurality of fixed nodes. Such location systems based on time of arrival and/or received signal strength to or from a plurality of fixed nodes may be referred to as multilateration systems.
It is also known that location may be estimated based on measurements of angle of arrival of radio signals, which may be referred to as triangulation.
Location systems and methods that do not require a survey typically rely on assumptions about radio frequency propagation, for example that the radio channel follows a direct line of sight path, and that the signal is not attenuated by obstructions. While this may be a reasonable assumption in many environments, indoor environments such as a typical office or business premises may involve severe multi-path, obstructions, and non-line of sight propagation, so that the accuracy of such location systems may be severely degraded.
Although satellite navigation systems such as GPS may be employed for location purposes outdoors, such systems tend to be less effective indoors due to the difficulty in receiving signals transmitted from satellites.
To enable location estimation in environments with unpredictable radio propagation characteristics, typically indoors, it is known to implement a system based on measurements of the radio frequency environment in terms of the strength of signals received at a terminal from a plurality of access points. The location is estimated by comparison of the measured received signal strengths with a database of received signal strengths from the access points, previously surveyed at a number of different locations throughout the area of coverage of the network. The measurements of received signal strength represent a “fingerprint” of a location in the network, that is to say a characteristic measure of the radio frequency environment of the location. However, location estimation by this method may suffer from ambiguities, in that a similar fingerprint may occur at several locations. The accuracy of the system may also be limited.
It is an object of the present invention to provide a method and apparatus which addresses these disadvantages.